Context. The formation of massive stars is a highly complex process in which
it is unclear whether the star-forming gas is in global
gravitational collapse or an equilibrium state supported by
turbulence and/or magnetic fields. In addition, magnetic fields may
play a decisive role in the star-formation process since they
influence the efficiency of gas infall onto the protostar.

Aims. By studying one of the most massive and dense star-forming regions in
the Galaxy at a distance of less than 3 kpc, i.e. the filament
containing the well-known sources DR21 and DR21(OH), we attempt
to obtain observational evidence to help us to discriminate between
these two views.

Methods. We use molecular line data from our 13CO 1 0, CS 2 1,
and N2H+ 1 0 survey of the Cygnus X region obtained with the
FCRAO and high-angular resolution observations in isotopomeric lines
of CO, CS, HCO+, N2H+, and H2CO, obtained with the IRAM
30 m telescope, to investigate the distribution of the different phases
of molecular gas. Gravitational infall is identified by the presence
of inverse P Cygni profiles that are detected in optically thick lines,
while the optically thinner isotopomers are found to reach a peak
in the self-absorption gap.

Results. We observe a complex velocity field and velocity dispersion in the
DR21 filament in which regions of the highest column-density, i.e., dense
cores, have a lower velocity dispersion than the surrounding gas and
velocity gradients that are not (only) due to rotation. Infall
signatures in optically thick line profiles of HCO+ and 12CO
are observed along and across the whole DR21 filament. By modelling the
observed spectra, we obtain a typical infall speed of ~0.6 km s-1 and mass accretion rates of the order of a few 10-3 yr-1 for the two main clumps constituting the filament.
These massive clumps (4900 and 3300 at densities of around
105 cm-3 within 1 pc diameter) are both gravitationally
contracting (with free-fall times much shorter than sound crossing times
and low virial parameter α). The more massive of the clumps,
DR21(OH), is connected to a sub-filament, apparently “falling” onto
the clump. This filament runs parallel to the magnetic field.

Conclusions. All observed kinematic features in the DR21 filament (velocity field,
velocity dispersion, and infall), its filamentary morphology, and the
existence of (a) sub-filament(s) can be explained if the DR21
filament was formed by the convergence of flows on large scales and is
now in a state of global gravitational collapse. Whether this
convergence of flows originated from self-gravity on larger scales or
from other processes cannot be determined by the present study. The
observed velocity field and velocity dispersion are consistent with
results from (magneto)-hydrodynamic simulations where the cores lie
at the stagnation points of convergent turbulent flows.

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